Document FR 02 817190 describes a master cylinder for a braking system comprising pistons made of plastic material and produced by molding. Each of the pistons divides a first and a second hydraulic circuit into a supply chamber and a pressure chamber, the pressure chamber being in communication with the supply chamber at rest through a duct made in the piston and blocked off by a shutter during a braking action.
The shutter and the duct form a valve, said valve being actuated by the displacement of the piston caused by an effort applied to the brake pedal. The supply chamber is connected to a brake fluid reservoir and the pressure chamber is connected to brakes arranged at the wheels of the motor vehicle. The valve is open in the rest position by means of a bearing element against which the shutter bears and is then lifted from a valve seat formed around the duct. In the primary circuit, the bearing element is a floating pin which is displaced with the movement of the piston. By contrast, in the secondary hydraulic circuit, the pin is fixed and mounted in the body of the master cylinder.
During a maximum travel of the secondary piston, its front end, that is to say its end directed away from the brake pedal, bears against the end wall of the master cylinder body. It has been observed that the action of the front end of the secondary piston bearing against the end wall of the master cylinder body can cause damage to the front end of the secondary piston, which is detrimental to reliable operation of the master cylinder.
Specifically, the plastic material used for the production by molding of the secondary piston in particular is not able to withstand repeated impacts. However, the economic advantage it provides on account of the simplicity of production of the piston by molding and on account of the weight saving provided thereby for the whole master cylinder, and therefore for the whole braking system, is very considerable. It is consequently desirable to be able to continue to use pistons made of plastic material in the master cylinders.
It would be conceivable to produce plastic pistons capable of withstanding the repeated impact against the body of the master cylinder by employing plastic materials of high mechanical strength. However, these plastic materials have a relatively high cost price and are much more difficult to use, which would cancel out a large part of the advantageous effect gained from employing the plastic material to produce the pistons of the master cylinder.